feat: Add the git_metrics module (#2827)

This PR adds a new module named git_metrics. It shows the added/deleted lines in the current git repository following the format: "[+$added_lines]($added_style) [-$deleted_lines]($deleted_style)".

src/configs/git_metrics.rs

@@ -0,0 +1,23 @@
+use crate::config::ModuleConfig;
+
+use serde::Serialize;
+use starship_module_config_derive::ModuleConfig;
+
+#[derive(Clone, ModuleConfig, Serialize)]
+pub struct GitMetricsConfig<'a> {
+    pub added_style: &'a str,
+    pub deleted_style: &'a str,
+    pub format: &'a str,
+    pub disabled: bool,
+}
+
+impl<'a> Default for GitMetricsConfig<'a> {
+    fn default() -> Self {
+        GitMetricsConfig {
+            added_style: "bold green",
+            deleted_style: "bold red",
+            format: "[+$added]($added_style) [-$deleted]($deleted_style) ",
+            disabled: true,
+        }
+    }
+}